The present invention relates to bulk material sampling systems and more particularly, to systems for extracting samples of bulk material from a moving conveyor belt.
Bulk material such as particulate coal or ore that is transported in a continuous stream by a belt conveyor is often the subject of cross-stream sampling. The usual objective of such cross-stream sampling is the characterization of certain material properties for the purposes of quality and value determination. The quality and character of a material sample frequently determines the value or use of that material deposited along the belt within a reasonable proximity of the sample. Consequently the statistical accuracy of sample representation is extremely important.
One example of equipment for extracting a material sample from the moving surface of a belt conveyor comprises a rotating sample cutter that is positioned to be driven about a rotational path that crosses the conveyor belt path. As the cutter transversely engages the material stream carried by the moving belt, that segment of material in the belt stream located between the rotational planes of the cutter side walls is swept or scooped into the cutter bin and becomes the sample increment.
As a general principle, the more rapidly the cutter passes through the material flow stream, the more accurate and reliable is the sample representation. The specification of U.S. Pat. No. 5,767,421, the disclosure of which is hereby incorporated by reference, describes the mechanics of how a rapidly moving cutter extracts a more complete and valuable representation of the material carried by the belt.
An independent incentive for greater cutter speed is the mathematical interrelationship between the width of the cutter opening, the material particle size and the belt speed. This interrelationship is described with respect to FIGS. 1A through 1C and is predicated on an empirical principle of statistical sampling that the effective cutter opening width, i.e. measured parallel with the belt traveling direction, should be between 2.5 to 3 times the width of the largest particle in the loose agglomeration of material particles that are the subject of the sample. FIG. 1A represents an increment of particulate material 10 deposited in an elongated pile on a stationary carrier belt 12. The material extraction swath 14a is that width of material removed from the elongated pile by passage of the cutter through the pile. The rotational planes 16a and 16b respective to the cutter sidewalls define the maximum width dimension of the swath 14a. If the width of swath 14a is 3 times greater than the effective diameter of the largest particle in the material pile 10 when the belt is stationary as represented by FIG. 1A, the normal width of swath 14b is reduced to 2.57 times the particle diameter when the cutter, moving at 1000 ft/min, traverses a belt 12 moving at 600 ft/min as illustrated by FIG. 1B. The normal motion vectors define a resultant angle of 59xc2x0 between the cutter side planes 16a and 16b and the belt traveling direction. This 59xc2x0 resultant angle reduces the normal width of the swath 14b to only 2.57 times the particle diameter.
FIG. 1C shows that for the same cutter moving at a speed of 700 ft/min traversing the belt moving at 1000 ft/min, the resultant vector angle is 35xc2x0 and the normal swath width 14c is only 1.72 times the particle size: an unacceptably low ratio.
Larger rotary belt samplers are usually driven by a pair of double-acting pneumatic cylinders. The cutter structure is secured for 360xc2x0 rotation about an axis or axle shaft. The axle shaft passes through two bearing supports. The cutter rotates in a swath between the supports. Outside of the cutter rotational swath, usually on opposite sides of the cutter swath, are a pair of crank throws radiating from the cutter axle shaft. The crank throws are structurally rigid with the axle shaft and cutter and angularly offset about the axle shaft axis whereby one rotatively leads the other about the axle shaft axis by about 20xc2x0 to 50xc2x0; usually about 40xc2x0. Each of these crank throws is pivotally connected to the rod end of a respective double-acting, single rod cylinder. The bore end of the cylinder unit usually is pivotally secured to a stationary axis aligned substantially parallel with the cutter rotational axis.
Characteristic of a double-acting cylinder is the use of pressurized fluid, i.e. air, on both sides of a piston that is structurally secured to a rod. Consequently, the cylinder has two pressure chambers. In those machines having the rod projecting from only one side of the piston, i.e. a single rod cylinder, these two chambers may be further distinguished as the head chamber and the rod chamber. This distinction is significant due to respectively different working areas and volumes. The working area and volume of the rod chamber is less than that of the head chamber by the area and volume of the rod that is stroked by displacement of the piston along the cylinder bore. The operative result of these distinctions is that greater fluid pressure is required in the rod chamber than the head chamber to produce the same rod force. However, more fluid volume is required in the head chamber than the rod chamber to produce the same rod displacement distance. Due to the many characteristics of a compressible fluid that effect the mass, density and flow rate across an orifice or through a port aperture, the optimum valve timing for opening and closing fluid flow ports respective to the head and rod chambers of a double acting cylinder is different.
Valve timing is generally referenced to the physical position of the piston within the cylinder bore when fluid is admitted to or released from a chamber. For example, when the rod is extended from the cylinder to the maximum, the piston position may be characterized as the upper dead center position. The opposite extreme, when the rod is drawn into the cylinder to the maximum degree, is characterized as the lower dead center position. Intermediate positions are characterized as before or after a center (upper or lower) position. If the outer end of the rod is connected to a crank throw, such intermediate positions may be designated in terms of crank rotational degrees.
Traditionally, fluid flow respective to the two chambers of a double-acting cylinder that drives a crank throw about a full 360xc2x0 rotation is controlled by a single xe2x80x9c4-way valvexe2x80x9d. The xe2x80x9c4-wayxe2x80x9d reference is to the number of conduit connection ports in the valve body. Two ports are dedicated to the fluid flow respective to the two pressure chambers in the cylinder. One valve port is dedicated to the fluid pressure supply and the fourth valve port is dedicated to the spent fluid discharge e.g. atmosphere. Operation of the valve xe2x80x9cspoolxe2x80x9d connects the pressure supply alternately with the rod chamber and the head chamber. Simultaneously, when the rod chamber is connected with the fluid supply, the head chamber is connected with the fluid discharge. No relative timing in the sequence of these four events is possible. The usual prior art practice, for example, is to switch the 4-way valve spool when the piston reaches upper dead center and switch again when the piston reaches lower dead center.
The consequence of an invariant prior art valve sequence is exacerbated by the structural fact that all working fluid must pass through the same 4-way valve twice in a rod stroke cycle. In effect, the fluid flow port of each chamber is as long as the respective conduit that connects the 4-way valve. The volume of each chamber is increased accordingly. Hence, when the valve spool shifts, pressurized fluid must first negate the inertia of the out flowing fluid to begin the fluid inflow. Sufficient fluid must thereafter enter the chamber flow port conduit to raise the pressure along the conduit before any force is transferred to the piston face. Simultaneously, when the valve spool opens a conduit to atmosphere, the pressure in the conduit, when opened, is at a maximum. Hence, the high pressure air in the conduit must be released before the pressure force on the piston face opposing opposite piston and rod movement is reduced.
Each of these transitional events require time and a discreet portion of the operational cycle. Moreover, because of the rod area/volume differential in a double-acting, single rod cylinder, the times required for charging and venting respective to the two chambers of a cylinder are not identical. Hence, twice in each rod cycle, the force flow from the rod declines thereby resulting in the rotational speed reduction of a belt sample cutter.
It is, therefore, an object of the present invention to optimize the valve timing for cutter drive cylinders.
Another object of the invention is a means to increase the rotational speed of a belt sample cutter: preferably without a supply pressure increase.
Also an object of the invention is a more efficient breathing system for double-acting pneumatic cylinders.
An additional object of the invention is provision for independent timing control over the respective pressure chambers of a double-acting cylinder.
A further object of the invention is the provision of minimum length fluid flow conduits respective to the pressure chambers of a double-acting cylinder.
Another object of the invention is a means to abruptly stop rotation of a sample cutter at a predetermined position around the cutter rotational circle.
Another object of the invention is a means to selectively reverse rotate the cutter to a predetermined park position around the cutter rotational circle and secure the cutter at the park position indefinitely.
Another object of the invention is a means to selectively reverse rotate the cutter to a predetermined park position around the cutter rotational circle and secure the cutter at the park position indefinitely.
These and other objects of the invention as will emerge from the following detailed description of the preferred embodiments are achieved by providing the double acting, single rod drive cylinders for a rotary sample cutter with closely coupled, 3-way valves that, preferably are solenoid actuated. Such a 3-way valve or the equivalent is connected in close proximity to each of the opposite pressure chambers in a single cylinder. The common port of each 3-way valve is connected, preferably by a close nipple, to the fluid flow port respective to a single pressure chamber. The discharge port of each 3-way valve is preferably open directly to atmosphere through a minimum length of conduit. The pressure supply port of one 3-way valve respective to one pressure chamber may be connected in parallel with that of the other 3-way valve respective to the other pressure chamber in the same cylinder.
The timing of each valve is individually triggered by a corresponding proximity switch that engages a solenoid to open and close the 3-way valve for one pressure chamber to pressure supply without regard to the status of pressure supply to the other pressure chamber.
Accordingly, the pressure supply may be opened to a pressure chamber while the piston is still approaching the upper dead center position. Also, the opposite, expanding pressure chamber may be opened to atmosphere prior to the piston lower dead center position. However, the two changes in fluid flow control need not occur simultaneously since the volume differential between the head chamber and the rod chamber will occasion different mass flow rates and inertia.
In brief, both chambers may be simultaneously pressurized or vented to atmosphere at operator discretion. Hence, the drive cylinders may be operated in a cutter braking mode as well as a rotational drive mode. Moreover, the rotary sample cutter may be rotationally reversed, selectively positioned and positionally held.